The present invention relates to switchable inductors for strong currents, namely for currents of several tens or even several hundreds of amperes. The invention has been developed in the context of antenna tuning circuits.
When an antenna is used for a transmission during which frequency hops are made, it is tempting to use a selective antenna and to modify its tuning at each frequency hop so that the transmission is always done under optimum conditions. This is the case, for example, in transmission with multiple-shift keying (MSK), i.e. transmission using modulation with hops from a frequency Of-dF to a frequency Of-dF and vice versa. Furthermore, the invention has been originally designed for transmission in MSK mode, at frequencies of the order of 15 kHz and with transmitter power values of the order of several hundreds of kilowatts.
Under such conditions of operation, attempts have been made to carry out the tuning with the primary winding of a tranformer with a saturable inductor, the value of the inductance of this transformer being adjusted by means of a direct current source connected to the terminals of the secondary winding and controlled at the pace of the MSK modulation. Unfortunately, this type of device generates high level harmonics owing to the fact that the switchable inductor thus obtained works at points where its value does not vary linearly as a function of the radiofrequency current. Now these harmonics are very difficult to filter.